In the case of a heavy duty tire for example, its tread outline shape a is formed into a single arc shape in a vulcanizing metal mold as schematically shown in FIG. 3.
In such a tire, however, in a standard internal pressure state in which the tire is assembled into a rim and a standard internal pressure is charged into the tire, there is a tendency that a tread surface swells radially outward in a region Y which is separated from a tire equator by a distance of 0.5 to 0.7 times of a tread half width. For this reason, a difference in circumferential lengths between a swelling portion b and a tread grounding end e is increased, slip between the tread surface on the side of the tread grounding end and a road surface is generated, and uneven wear such as so-called shoulder wear is prone to be generated.
In order to restrain the shoulder wear, Japanese Patent Application Laid-open No. H7-164823 and the like disclose a technique in which the outline of the tread in the vulcanizing metal mold is formed into a double radius shape in which a portion of the tread on the side of the tread grounding end is an arc having a greater radius of curvature than a portion of the tread on the side of the tire equator, thereby bringing the outline of the tread in the standard internal pressure state closer to the single arc to restrain the shoulder wear.
Such a technique can restrain the shoulder wear in some degree but there is a problem that new shoulder wear is produced in the region Y. Especially when a circumferential groove is formed in the region Y, there is a tendency that so-called railway wears in which the groove side edge of the circumferential groove wears are seriously generated.
It is conventionally known that there is a correlation between a grounding surface shape and an uneven wear of a tire, and a slip is generated in a portion of the tire in which the grounding length is shortened and wear proceeds fast. Thereupon, the present inventor researched that the grounding surface shape was improved to suppress the uneven wear such as shoulder wear and railway wear, thereby equalizing the wear.
As a result, the inventor found that the grounding surface shape has a correlation between a tread thickness from a second belt ply to a tread surface in a belt layer and a tread thickness from a carcass to the tread surface, and if each of the tread thickness is limited, the grounding surface shape could be optimized.
That is, it is an object of the invention to provide a heavy duty tire capable of improving the grounding surface shape and equalizing the wear based on an idea that the tread thickness from the second belt ply and the tread thickness from the carcass are limited.